This invention relates to a method and apparatus for forming an injection molded plastic article having amorphous and crystallized portions generated from the melt phase of the plastic resin as well as a novel molded plastic article. More particularly, the invention relates to a novel partial crystallization method and apparatus conducted entirely or partially inside a mold which is particularly applied to the injection molding of novel polyethylene terephthalate (PET) preforms having a crystallized neck portion and an amorphous body portion. This type of preform is further blown into a biaxially oriented container with improved strength properties in the desired crystallized area. The invention is applicable to preforms made of a single or multiple materials. The previous injection and blow molding methods for PET preforms show crystallized portions generated from the glass phase by local heating and then cooling. The present invention teaches a preferential cooling temperature pattern applied when the article is in a melt phase whereby an amorphous portion is created by rapid local cooling and a crystallized portion is created by a relatively slower local cooling.
Preferentially, the present invention uses novel designs of the PET preform molding and handling equipment. The present invention can also form other kinds of plastic articles exhibiting areas of different physical or optical characteristics. A preferred example in this regard is represented by innovative information carrier plastic discs that have a central hole, such as the HD""s, CD""s, DVD""s, MOP""s and CD-ROM""s which have a central area without retrievable information. Especially the CD""s and DVD""s withstand critical manipulations in use and thus have to have enhanced strength and stiffness characteristics in their central portion. Even higher strength and stiffness characteristics are essential for injection molded precision and high torque resistant plastic gears actuated by rotation shafts.
It is well known that polyethylene terephthalate (PET) blown articles exhibit biaxial-orientation and thus have excellent properties which make them suitable for many applications. However, the biaxial orientation of a hollow shaped article has the disadvantage of strongly increasing the internal stress, thereby causing a reduction of dimensional stability under heat in excess of 70xc2x0 C. Of critical importance in many packaging applications is the neck portion of a container which is not biaxially oriented during the blow-molding process and thus is weaker than the rest of the part. One common solution to strengthen the neck portion of an amorphous PET parison or bottle is to crystallize the neck portion by local heating at a temperature of more than 140xc2x0 C., where a rapid spherullitic crystallization of the non-oriented amorphous material occurs. Many attempts have been made to improve the strength of the neck portion of PET blown bottles by local reheating of the preform or of the bottle, after injection molding of a blowable PET parison. Reference is made in this regard to U.S. Pat. No. 4,375,442 to Ota, U.S. Pat. No. 4,589,559 to Hayashi, U.S. Pat. No. 4,928,835 to Collette, U.S. Pat. No. 4,933,135 to Horwege, U.S. Pat. No. 5,180,893 to Sugiyama and U.S. Pat. No. 5,248,533 to Sugiura. In all these approaches, a heating source is directed towards the neck portion of a molded preform which is heated to a temperature necessary for its crystallization and heat set. A different approach to form a preform with a crystallized neck is disclosed in EP 503086 to Orimoto et al. In this case, a molded neck finish is first molded and then crystallized outside the mold and then located in a second mold as an insert to form a complete parison by an overmolding method.
Crystallization of the neck finish of a molded parison or a molded neck from glass does not represent an optimum method because it represents an additional, time consuming step to make a heat resistive blown article. Furthermore, it requires additional equipment and floor space and is highly energy inefficient. In most cases, the reheated and crystallized neck does not preserve its nominal dimensions.
Attempts have been made in the past to control crystallization in thermoplastic plastic materials during the injection molding process by varying the pressure on the thermoplastic material in a controlled manner during the cooling cycle. This method, disclosed for example in U.S. Pat. No. 4,150,079 to Chang, is hardly implementable when it comes to crystallize only a limited portion of the article, as is the case with the neck portion of a preform.
Partial crystallization has been also applied to vacuum thermoformed articles, as disclosed in the PCT application WO 88/09298 to Clarke. Clarke ""09298 fairly describes a vacuum mold or press for shaping a flat blank of molded PET into a food container with reinforced areas. The mold of Clarke has local heaters, local coolers and local thermal insulators to induce crystallized and amorphous areas of the already flat blank. Clarke ""09298 mold does not receive molten PET and does not use preferential cooling/heating of a molten material to form crystallized patterns. Therefore his method is not applicable to the injection molding of partially crystallized articles, such as PET preforms, by selective cooling from melt.
In-mold local crystallization during normal cooling has been disclosed in U.S. Pat. No. 4,307,137 to Ota. The subject matter of the ""137 patent is to create a visible difference in the body of a blown preform by creating an opalescent pattern during the in-mold cooling of molten PET when molding a preform. Thus, the ""137 patent is not interested in the local increase in the strength of a preform through in-mold crystallization. Also, the ""137 patent does not teach any new means to locally alter the cooling rate in a mold. What clearly appears from Ota""s disclosure (see FIG. 5 and FIG. 6) is that he uses the geometry of the molded part itself to achieve partial crystallization by using any standard well known molding equipment. Ota""s crystallization method is based on the inherent fact that a PET article in the molten state that has regions of variable thickness will require more time to solidify in the thick regions than in the thin regions at the same temperature. Accordingly, thick regions that resist cooling may become crystalline while the thin regions that cooled faster become amorphous during the normal cooling step of a PET melt. The ""137 patent creates the thin and thick areas in a two stage overmolding process. A first article comprising variable thickness areas is molded and then used as an inner xe2x80x9cmoldxe2x80x9d surface to form the final composite article where the second layer has corresponding thick and thin areas. During the normal cooling process the thick areas of the second outer layer have a slow cooling and thus crystallize while the thin areas of the second outer layer have a faster cooling and thus become amorphous. The crystallized areas are used as a design pattern. As mentioned before, this select cooling process of the ""137 patent is achieved in a regular mold solely due to the thickness variation of the second molded layer.
The crystallization method, apparatus and article of the present invention is different from the known methods because it uses a differential cooling pattern induced by innovative molding equipment to slow down the cooling process of the resin in a substantially molten phase only in certain selected areas. This local crystallization method that provides strength enhancement can be understood from the following brief description of the polymer""s behavior under heat.
Accordingly, it is a principal object of the present invention to provide an improved, injection molded plastic article and a process and apparatus for preparing same.
It is a further object of the present invention to provide an article, process and apparatus as aforesaid which includes a crystalline portion with enhanced strength properties, and an amorphous portion, especially having a crystallized neck portion.
Further objects and advantages of the present invention will appear hereinbelow.
In accordance with the present invention, the foregoing objects and advantages are readily obtained.
The process of the present invention comprises: forming a plastic article by injecting molten plastic into an injection mold at an elevated temperature; cooling the plastic in said injection mold from the molten condition to a solid condition; wherein a first portion of the plastic is rapidly cooled from the molten condition to the solid condition to form an amorphous portion, and a second portion of the plastic is slowly cooled from the molten condition to the solid condition to form a crystallized portion. Desirably, said article has a closed bottom, a sidewall portion extending from the closed bottom, and a neck region extending from the sidewall, including the step of crystallizing at least a portion of the neck region. In a preferred embodiment, the present invention includes the step of providing cooling means in the injection mold to cool said article, and insulating at least a portion of the neck region from said cooling means.
The apparatus of the present invention comprises: an injection mold having a mold cavity therein; means for injecting molten plastic into said mold cavity to form a plastic article therein; cooling means in heat exchange relationship with said mold cavity to cool said plastic from the molten condition to the solid condition; wherein said cooling means is operative to rapidly cool a first portion of said plastic from the molten condition to the solid condition to form an amorphous portion, and including means adjacent a second portion of said plastic to permit slowly cooling said second portion from the molten condition to the solid condition to form a crystallized portion. Desirably, the article has a closed bottom, a sidewall portion extending from the closed bottom, and a neck region extending from the sidewall, wherein the means adjacent the second portion is adjacent at least a portion of the neck region. In a preferred embodiment, the means adjacent the second portion insulates at least a portion of the neck region from the cooling means.
The injection molded article of the present invention comprises: an injection molded plastic article formed by injecting molten plastic into an injection mold, wherein said article includes an amorphous first portion formed by rapidly cooling said molten plastic to the solid condition, and a crystallized second portion formed by slowly cooling said molten plastic from the molten condition to the solid condition. Desirably, the article has a closed bottom, a sidewall portion extending from the closed bottom, and a neck region extending from the sidewall, wherein at least a portion of the neck region is crystallized. In a preferred embodiment, the article is a preform.
In accordance with the present invention, the foregoing readily obtains the objectives and advantages of the present invention. Further features of and advantages of the present invention will appear hereinbelow.